The present invention relates to an electric discharge detection circuit used in fluorescence x-ray analyzers.
An x-ray tube voltage applied to an x-ray tube can be monitored by resistance division of the output from a high voltage power supply used for an x-ray tube with a high resistance and a high withstand voltage. Whether there is an electric discharge phenomenon or not is determined by the detecting the electric voltage in proportion to the tube voltage obtained through the aforementioned method, i.e. a sharp drop in the monitored level of the X-ray tube voltage.
A filter circuit is typically used for eliminating high frequency noise originating from the x-rays. A sharp drop in the monitored tube voltage output level, which is input to the differentiation circuit through the filter circuit, is detected by comparing the voltage level outputted from this differentiation circuit with a specified reference voltage level by means of a comparator. However, the degree to which the monitored level of tube voltage drops is not stable, making it difficult to set constant values for the differentiation circuit and the reference voltage.
For example, when the constant used for differentiation is set to be small or the reference voltage is set too high, electric discharge phenomenon may not be detected if the extent to which the monitored tube voltage output level has dropped is relatively moderate. If the analyzer is used continuously for a long period of time without detecting any electric discharge, overload current that flows into high voltage circuits may damage various high voltage parts and units, including the connectors and the power supply. Moreover, the high frequency current that flows inside the device while the electric discharge taking place may have a detrimental effect on the workings of electronic circuits and may also damage the electronic parts.
On the other hand, when the constant used for differentiation is set to be large or the reference voltage is set at a low level, insignificant fluctuations in tube voltage that actually have no relation to electric discharge may be erroneously detected as an electric discharge phenomena. Furthermore, a variety of electrical and electronic parts are assembled in the fluorescence x-ray analyzer. If an output such as a surge caused by these internal parts is superimposed on the tube voltage monitor, the filter circuit located in front of the differentiation circuit may not be able to eliminate it, and as a result, the analyzer may misjudge it to be a cause of electric discharge phenomenon. If the analyzer responds hypersensitively to the changes in the monitored tube voltage output level, the safety circuit will be activated frequently and may stop the analyzer whenever x-rays are generated, thus reducing its measurement throughput capacity.
Thus, advantages of the following embodiments include preventing malfunctions caused by noise, and furthermore, providing a highly reliable electric discharge detection circuit for an x-ray analyzer.
To solve the problems described above, the present invention provides an electric discharge detection circuit, comprising: an x-ray tube; a power supply to generate a high voltage applied to the x-ray tube; an x-ray tube voltage detector to detect the high voltage applied to the x-ray tube; a differentiation circuit to differentiate the signal output from the tube voltage detector, a zero-crossing comparator that discriminates the polarity of the output signal from the differentiation circuit; a re-triggerable one-shot pulse generating circuit to generate a one-shot pulse at a regular period, using the pulse outputted from the zero-crossing comparator as a trigger; a counter having the one-shot pulse output from one-shot pulse generating circuit input as an operation enable signal, to count pulses output from the zero-crossing comparator while operation is enabled; an x-ray cut-off circuit to send a command signal to the power supply to stop the generation of high voltage when it receives a carry output from the counter; and a display to display the fact that an electric discharge phenomenon is occurring upon receipt of the carry output from the counter.
According to this structure, an electric discharge phenomenon that occurs in an x-ray generating system is displayed in the form of a pulse train. These pulses act as the trigger to generate one-shot pulses at a regular period that is input to the counter as an operation enable signal. The counter counts the pulses input while operation is enabled and outputs a carry when the pulse count reaches a preset value. The display receives this carry output displays the electric discharge phenomenon and notifies this fact to the user. An x-ray cutoff circuit that also receives this carry output simultaneously sends out a command to the power supply to switch off its high voltage output. Since the one-shot pulse generating circuit is re-triggerable, it will keep on outputting the operation enable signal to the counter as long as the pulse train is generated continuously within the set period. On the contrary, if only a single pulse is generated, the counter will not reach its xe2x80x9ccount-upxe2x80x9d state since it will be reset immediately after the expiration of the set period. As a result, only electric discharge phenomena that are persistent will be detected, thus allowing for prevention of electric discharge from causing critical damage to the analyzer.